Most metropolitan areas are now equipped with one or more forms of wireless communications networks which provide mobile telephone and other related services to customers across a broad frequency spectrum. Consider, for example, what has come to be known as "cellular" telephone services or Personal Communication Services (PCS), i.e., radio transmissions in the microwave band between approximately 800 MHZ and 2.2 GHZ. As shown in FIG. 1, prior art cellular telephone systems 10 include a Mobile Telephone Switching Center (MSC) 12 and a plurality of cell site transceivers 14a-14c. The cell site transceivers transmit radio signals to and receive radio signals from one or more mobile units 16 that move about a cellular service area 20. A mobile unit, as the term is used herein, refers to a wireless voice telephone or data receiver that can be permanently installed at a fixed location or within a vehicle or that can be portable. Each cell site transceiver 14 is able to broadcast and receive the radio signals within a geographic area 18 called the cell site coverage area. Together, the areas 18 comprise the entire cellular service area 20. Typically, a cellular service area comprises a metropolitan area or larger region.
When a telephone call to a called mobile unit 16 originates from either another mobile unit or a land-based telephone via a Public Switched Telephone Network (PSTN) 22, a caller must first access the cellular telephone system 10. This task is accomplished by dialing the mobile unit's unique identification number (i.e., its phone number). The MSC 12 receives the call request and instructs a central call processor 24 to begin call processing. The central call processor 24 transmits a signal over a dedicated line 26 (such as a telephone line or microwave link, etc.) to each of the cell site transceivers 14a-14c causing the cell site transceivers to transmit a page signal that the mobile unit 16 receives. The page signal alerts a particular mobile unit 16 that it is being called by including as part of the page signal the paged mobile unit's identification or phone number.
Each cell site transceiver 14 transmits the page signal on one or more dedicated forward control channels that carry all pages, as well as control signals, channel assignments, and other overhead messages to each mobile unit. The forward control channel is distinct from the voice channels that actually carry voice communications between a mobile unit and another mobile unit or a land-based telephone. Each cell site transceiver may have more than one forward control channel upon which pages can be carried.
When a mobile unit is not engaged in a telephone call, it operates in an idle state. In the idle state, the mobile unit will tune to the strongest available forward control channel and monitor that channel for a page signal or other messages directed to it. Upon determining that a page signal is being transmitted, the mobile unit 16 again scans all forward control channels so as to select the cell site transceiver transmitting the strongest signal. The mobile unit then transmits an acknowledgement signal to the cell site transceiver over a reverse control channel associated with the strongest forward control channel. This acknowledgement signal serves to indicate to the MSC 12 which of the forward control channels (associated with the several cell site transceivers 14a-14c) to use for further call processing communications with mobile unit 16. This further communication typically includes a message sent to the mobile unit instructing it to tune to a particular voice channel for completion of call processing and for connection with the calling party. The details of how the cell site transceivers transmit the signals on the forward and reverse control channels are typically governed by standard protocols such as the EIA/TIA-553 specification and the air interface standards for Narrowband Analog Mobile Phone Service (NAMPS) IS-88 and IS-95 air interface standards for digital communications, all of which are well known to those of ordinary skill in the wireless telephone communications art and therefore will not be discussed.
The EIA/TIA/IS-95 "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" defines a digital cellular radio common air interface. The standard makes use of Code Division Multiple Access (CDMA) technology, where both the base station and the mobile transmit a Pseudo-random Noise (PN) spreading sequence, resulting in a 1.23 MHz transmission bandwidth.
The forward link transmission from each base station consists of four types of channels: pilot, paging, synchronization (sync), and traffic. The channels are all transmitted on the same carrier frequency, using the same PN spreading code; however, the channels are distinguished through binary orthogonal codes based on Walsh functions. Each base station transmits 1 pilot channel, 1 sync channel, and multiple paging and traffic channels. Forward link signals from different base stations are distinguished through PN spreading code phase offsets. In other words, every base station uses the same PN spreading code, but the code is transmitted using different time offsets (i.e., code phases) from the master code. There are 512 allowable code phase offsets.
The CDMA reverse link from the mobile to base station are differentiated by the use of a long PN spreading code (2.sup.42 -1), where each mobile transmits at a code phase time offset determined by a user address. Before being assigned to a reverse link traffic channel, the mobile must contact the base station using a reverse link access channel. The mobile station receiver employs multiple demodulators, which allows simultaneous demodulation of signals from up to three base stations.
While wireless communication services have been quickly embraced by those people whose business requires them to travel frequently and to be in constant communication with their clients, many individuals are discouraged from utilizing present wireless telephone systems. The main reason for this discouragement is the relatively high price charged by the service providers for a wireless telephone call. In conventional land-based telephone systems, the price of a telephone call can be tailored to reflect the supply and demand of the telephone resources available to a given area. For example, in large metropolitan areas or financial districts, users can typically expect to pay more for a telephone call than in a residential area. With current wireless technology, however, available position data is strictly limited to relatively large coverage areas and sectors thereof as defined by the RF characteristics, i.e. "footprints" of the associated transceiver. As explained below, these coverage areas are generally unsuitable for most commercial and consumer applications.
With reference again to prior art cellular telephone systems of the type shown in FIG. 1, for example it is known by those skilled in the art that typical cell sites are divided into three "sectors" 27. Each of these sectors 27 operates as a nearly independent cell site in that it is independently controlled by the associated switch. In operation, the wireless service provider must identify user location to the coverage area of one or more of these cell sites or sectors of cell sites with the associated dependence on erratic RF propagation as a definition tool. Cell site coverage areas and sectors vary widely in size. However, they are typically on the order of several square miles in less populated rural and residential areas and decrease accordingly in more densely populated metropolitan areas. Against this background, there have been previous attempts to provide methods and systems which generally identify the positions of wireless communication system users in the above cell site coverage areas and sectors. See, for example, U.S. Pat. No. 4,876,738 issued to Selby and assigned to U.S. Philips Corporation. Selby discloses a registration procedure in which the base station monitors the location of the mobile unit by cell site. The effect is to allow enlargement of the registration area if the mobile unit consistently roams between two cells.
See also, U.S. Pat. No. 5,179,721 issued to Comroe et al and assigned to Motorola Inc. Comroe discloses a method for inter-operation of a cellular communication system and trunking communication system by transmitting an access number for each system such that the mobile unit may be used as a cellular telephone and a trunking communication device.
Finally, see U.S. Pat. No. 5,097,499 issued to Consentino and assigned to AT&T Bell Laboratories. Consentino teaches a method for preventing an overload in a reverse channel by delaying the time of the generation of timing stamps on markers.
These methods and systems, however, have proven unsuitable for commercial and consumer applications where users may, at any given time, travel through very small portions of numerous cell site coverage areas and sectors. Consider, for example, a wireless telephone customer who resides in Boulder, Colo. and who commutes to work on a daily basis to downtown Denver. The customer may typically place and receive wireless telephone calls only in the approximate 45 mile geographic area between the two cities during her commute to and from work. In fact, the customer may not even wish to be provided telephone service outside of this limited geographic area. Alternatively, she may wish to restrict telephone service outside of this area to selected hours on weekends and evenings only. This desired service area, however, most likely spans small portions of the numerous cell site coverage areas, the latter portions of which the customer has no desire of using and does not wish to be billed for. However, under current wireless technology, and as described in the prior art referenced above, presently available positioning methods and systems are limited to a determination of whether the user is within one or more predetermined cell site coverage areas or sectors. These prior art systems are incapable of providing further detail, i.e. exactly where in the cell site coverage area the user is located. The prior art systems, which might be implemented in this example to determine that the customer has passed through one or more large cell site coverage areas, does not meet the objectives of the customer and is thus not a commercially marketable product.
Prior art attempts to design higher accuracy positioning systems which utilize commercial broadcast transmissions, for example, have also met with limited success. See, for example, U.S. Pat. Nos. 4,054,880 (Dalabakis et al) and 3,889,264 (Fletcher) which disclose what are known as "Delta-Position" systems. These prior art patents describe systems using three spectrally spaced-apart radio signals, each of which is an independent AM radio signal. The systems typically have a vehicle carried mobile receiver, with a separate tuner for each station, and a second receiver at a fixed, known position. As disclosed, these systems count "zero crossings", each of which indicates that the user has moved a certain distance from his or her previous location. In operation, if a user requires knowledge of his or her current position, the user must first specify a starting position. A fixed position receiver detects frequency drift of the transmitters, which is used to adjust and coordinate the zero crossing counts made by the mobile receivers.
These systems are termed "Delta-Position" systems because they determine only the distance and direction traveled by a mobile user from any particular starting point. Neither Dalabakis nor Fletcher actually determines the position of the mobile user.
See also, U.S. Pat. No. 5,173,710 to Kelley et al which discloses the use of a fixed position receiver which is adapted to determine frequency drift along with the relative phases of various unsynchronized FM broadcast signals originating from known fixed locations. As disclosed by Kelley, each of the fixed transmitters transmits a beacon signal having a phase that is unsynchronized with the phases of the beacon signals of the other transmitters. These signals are 19 kilohertz analog pilot tones generated by commercial broadcast stereo FM stations. The fixed receiver receives the beacon signals, determines the relative phases of the beacon signals, and broadcasts data representing these relative phases for receipt by the mobile receiver which is at an unknown location. Each mobile receiver includes phase measurement circuitry that detects the phases of the beacon signals at the mobile receiver's current position on multiple distinct carrier frequencies such that the current position of the mobile unit may be determined when used in conjunction with the fixed receiver broadcast data.
None of the systems referenced above, as well as general time difference of arrival location systems such as LORAN, NAVSTAR, and GPS, as used, for example, in U.S. Pat. No. 4,833,480 issued to Palmer et al, have proven suitable for commercial applications since, by design, they require specially adapted receivers to receive and process the pilot tones, GPS signals, etc. at the mobile unit. This sophisticated end equipment, of course, significantly adds to the cost of the corresponding mobile unit. In the case of hand portable units, this additional equipment further results in a handset which is extremely bulky and difficult to handle. As a result, these systems have proven unsuitable for both large-scale commercial applications, as well as ordinary consumer use.
When applied to wireless communications of interest to the present invention, i.e., communications in the microwave band from 800 MHZ to 2.5 GHZ, these prior art systems are further considered unsuitable for commercial applications in view of their anticipated use of excessive frequency spectrum. More specifically, it is anticipated that for proper operation these systems would necessarily require transmission of signals on separate channels which would utilize an unacceptable amount of additional spectrum.
Consequently, a need has developed to provide a positioning system and method which may be practically and economically implemented for use in wireless communications systems and, in particular, in the microwave band from 800 MHZ to 2.5 GHZ. Still further, a need has developed to provide such a positioning system which may be used by wireless telephone customers to determine, in advance, customized service zones for billing and access purposes. Still further, a need has developed to provide such a positioning system and method that allows a user to determine whether she is in one of the predefined customized zones within the wireless communications service area and to convey that information to the wireless communications service system. It is further desirable that such a system and method be compatible with existing wireless telephone technology and should not degrade the operation of an existing system. Finally, such a system should neither require the allocation of more radio frequencies than are currently allocated to wireless telephone systems, nor require a substantial portion of existing wireless frequencies.